The trend toward optimizing equipment design to achieve the maximum capability of equipment with relationshp to weight, size, and economy of material usage have spurred considerable activities in the area of fastener tension and inspection methods. A considerable amount of the early development work centered on torque control as a means of fastener tension. However, the accuracy of this method is severely limited by its sensitivity to such factors as thread condition and other factors affecting the coefficient of friction.
To minimize the effect of friction, a later development, often referred to as the so-called "turn of the nut" method, was evolved. The method prescribed a combination of torque (to assure the fastener was seated) and rotation (using the thread of the bolt as a micrometer to stretch the bolt). This method achieves considerable accuracy in tensioning the bolt under carefully controlled fastener and joint system conditions. However, the torque controlled starting point often leads to difficulties by false starts (the fastener or the joint system not properly seated or because of thread condition causing high prevailing torque).
An even more recent development is the method of bringing the bolt to its recognizable yield point (a well-defined point of tension) and utilizing that point to ultimately arrive at the desired bolt tension either by memory of the tightening cycle or an "unturn of the nut" method. While these later methods result in reasonably accurate bolt tension, the methods have some draw backs in universal application. In many applications, it is not desirable to bring the fastener to its yield point. The joint may not be capable of sustaining the full tension of a yielded fastener without damage such as flange warpage, gasket crushing, or thread failure.